1. Field of the Invention
The present invention relates to a projection type video display, such as a liquid crystal projector, including a light source lamp and a light valve to display a video image by modulating the brightness of the light source lamp.
2. Description of the Related Art
High-pressure discharge lamps have a high brightness feature and are commonly used as lamps for projection type video displays, such as liquid crystal projectors.
However, a problem with conventional high pressure discharge lamps is that the movement of discharge position causes fluctuation in lamp brightness and degradation of image quality as a result of flicker in the projection image.
It is well known to prevent this movement of the discharge position in the conventional high pressure discharge lamp by using the lamp, which is set at the rated electrical power per arc length, as a light source lamp in the projection type video display, and applying to the lamp an alternating voltage having a frequency causing minimal change the brightness of the lamp (for example JP 2000-323287).
However, the temperature of the lamp, the alteration of an electrode shape and the fluctuation of voltage applied are likely to cause the movement of the discharge position. If the discharge position is moving, it has the drawback of causing the fluctuation of the projection image continuously because its state isn't able to be detected and the position moved isn't returned to the former position.
If you exchange the specified lamp for a general lamp at the end of its effective duration, the specified frequency doesn't have any effect on a general lamp with respect to reducing the movement of discharge position in the general lamp. Therefore it also has the drawback of causing the movement of discharge position.